There is an ongoing need for replacement materials for cancellous bone, particularly where such materials are cell and tissue, receptive. It is desirable that cancellous bone replacement materials provide a porous framework allowing for revascularization as well as new bone growth, and one which provides a compatible site for osteoprogenitor cells and bone growth-inducing factors. Grafting, however, requires surgery to obtain natural material, and a viable substitute synthetic material is desirable. Thus, a suitable, synthetic cancellous bone replacement material would be beneficial to these ends. In order to mimic the behavior of cancellous bone grafts, it is expected that the physical characteristics of this material should be reproduced in the synthetic material. Thus, any such material should be strong, biocompatible, should match the biomechanical requirements and performance of the natural material and have a porous framework which promotes revascularization and bone regrowth. For these latter two processes to occur, it is critical that bone ingrowth into and onto the replacement material occur to an appreciable extent.
The voids and interstices of a porous material provides surfaces for bone ingrowth, thereby providing ideal skeletal fixation for the permanent implants used for the replacement of bone segments lost due to any number of reasons, or in total joint prostheses. The implants may be conventional total joint replacements such as artificial hip, knees, etc., or partial joint replacements such endoprostheses components. A number of characteristics are known in the art to be important. These include porosity, biological compatibility, intimate contact with the surrounding bone, and adequate early stability allowing for bone ingrowth. The ideal porous replacement material should have good strength, especially good crack and impact resistance and a compliance comparable to that of bone. The material should be ideally be amenable to the easy and simple manufacture of implants of precise dimensions, and permit the fabrication of either thick or thin coatings on the materials.
One important requirement for successful ingrowth is that the implant material be placed next to healthy bone. In fact, the presence of bone within the implant demonstrates the osteoconductive, or bone-growth promoting properties of the porous structure of the implant when it is placed in physical contact with healthy bone tissue. Initially, the cells that interface the implant convert to bone, then the front of regenerated bone progresses into the implant.
There have been numerous efforts to develop and manufacture synthetic porous implants having the proper physical properties required to promote bone ingrowth. Implants with porous surfaces of metallic, ceramic, polymeric, or composite materials have been studied extensively over the last two decades.
The most commonly used substance for porous biomaterials is calcium hydroxyapatite (HA), which is the largest chemical constituent of bone. Other nonmetallic materials frequently used in porous form for implants include the ceramics tricalcium phosphate (TCP), calcium aluminate, and alumina, carbon; various polymers, including polypropylene, polyethylene, and polyoxymethylene (delrin); and ceramic-reinforced or -coated polymers. Unfortunately, ceramics, while strong, are very brittle and often fracture readily under loading; and polymers, while possessing good ductility, are extremely weak. The very nature of these materials can restrict their clinical dental and orthopedic applications.
Metals, on the other hand, combine high strength and good ductility, making them attractive candidate materials for implants (and effectively the most suitable for load-bearing applications). Many dental and orthopedic implants contain metal, most often titanium or various alloys such as stainless steel or vitallium (cobalt-chromium-molybdenum). Ceramic-coated metals are also used, typically HA or TCP on titanium. Additionally, a large variety of metals are used internally in biomedical components such as wire, tubing, and radiopaque markers.
Many existing metallic biomaterials, however, do not easily lend themselves to fabrication into the porous structures that are most desirable for bone implants. These materials (e.g. stainless steel, cobalt-based alloys) exhibit the necessary properties and biocompatibility as long as only a smooth, bulk shape in a metallurgically perfect state is needed. The machining or other treatment needed to obtain a porous or surface-textured shape for interlocking with skeletal tissue can have a detrimental effect on the properties and biocompatibility, and can even result in material failure. For example, the hexagonal crystal structure of titanium makes it susceptible to cracks and fractures, as has been seen in the case of dental implants. Some porous metallic materials (e.g. flame- or plasma-sprayed titanium, porous sintered powder metallurgy materials) do not match the structure of cancellous bone sufficiently well to ensure successful ingrowth and integration. Also, most metals and alloys currently in use are subject to some degree of corrosion in a biological environment. Finally, the high densities of metals can make them undesirable from a weight standpoint.
A significant step in the improvement of porous implants occurred with the introduction of a reticulated open cell carbon foam is infiltrated with tantalum by the chemical vapor deposition (CVD) process that was described in U.S. Pat. No. 5,282,861. The '861 patent taught a new biomaterial that, when placed next to bone or tissue, initially serves as a prosthesis and then functions as a scaffold for regeneration of normal tissues. The '861 material fulfills the need for an implant modality that has a precisely controllable shape and at the same time provides an optimal matrix for cell and bone ingrowth. The physical and mechanical properties of the porous metal structure can be specifically tailored to the particular application at hand. Although it is expected to have its greatest application in orthopedics, this new implant material offers the potential for use in alveolar ridge augmentation, periodontics, and other applications. As an effective substitute for autografts, it will reduce the need for surgery to obtain those grafts.
The open cell structure of the prior art is made from tantalum. Most of the current orthopaedic implants are made from titanium or cobalt chromium alloy, or more recently from zirconium alloy. The use of tantalum along with the titanium or cobalt chromium alloy poses a possibility of galvanic interaction, the effects of which are currently not known. A porous structure that is made from an alloy which is currently used in the orthopaedic industry will be a great advantage as it can be safely incorporated with the existing alloying system. However, the open cell structures of the prior art suffered from a lack of strength for certain implant applications. An improvement in the state of the art of porous implant structures may be achieved if the strength of the structure can be improved. This would facilitate its widespread use in both conventional implants such as hip, knees, etc., as well as in specialty applications such as replacements for vertebral bodies that make up the spinal column.